A consistent thermal and chemical non-equilibrium model for inductive supersonic plasma flow, developed recently, is applied to the modelling of pure argon supersonic plasma flow, which impinges on a substrate below the Mach 1 nozzle. The model considers the ionization of argon atom and the corresponding recombination but the second order ionization is ignored and plasma charge neutrality is assumed. The transport and mass diffusion coefficients are computed using the collision cross-section data, published by Devoto and Murphy and the computations of transport properties are fully coupled with the calculation of the plasma flow fields. The model treats the subsonic discharge region above the supersonic nozzle and the supersonic region below the nozzle together. Two different turbulent models are incorporated into the model to describe the supersonic plasma flow. The modeled radial and axial profiles of electron and heavy species temperatures and electron number densities near the substrate are then compared to those measured by the method of optical emission spectroscopy and finally the most realistic model is identified.

Ceramic functional coatings are frequently applied to structure materials in a wide range of mechanical and chemical applications for their specific properties of hardness, passive resistance to chemical aggression and low thermal conductivity. The counter part of those specific properties is a low mechanical and thermal shock resistance. Thus it is of prime importance to investigate the fundamental mechanisms, which governs the failure under thermal shock and thermal cycle conditions to increase the range of reliable industrial applications. The work presented in this paper deals with the Finite Element Modelling [FEM] of the transient thermal fields and associated stresses developed in ceramic coatings during firstly the post deposition cooling step and secondly during thermal cycling conditions. Alumina [Al 2 O 3 ] and Partially Stabilized Zirconia [PSZ] coatings, which are both widely used in mechanical and chemical industries, are under investigation.

A thermal and chemical non-equilibrium model is developed for the modelling of multi-component supersonic induction Ar-H 2 plasma flows. The species included in the modelling are electrons(e), hydrogen ion(H+), hydrogen atoms(H), hydrogen molecules(H 2 ), Argon ions(Ar+) and Argon atoms(Ar). The negative hydrogen ions(H-), molecular hydrogen ions(H 2 +) and second order ionisation are neglected. The chemical reactions considered in the modelling are the H 2 dissociations and the corresponding recombination, induced by Ar atom and H 2 , and the ionisations of the hydrogen and Argon and the corresponding recombination. All the heavy species are assumed to have the same temperature (Ti). The electron temperature (Te) is allowed to deviate from that of heavy species. The energies for these chemical reactions have been treated as the source terms for energy conservation equations. As a result, the contributions of these chemical reactions to the total enthalpy are removed. Therefore, the heavy species temperature can be obtained by solving the thermal kinetic energy equation, rather than the total enthalpy equation. Yos’s mixing law is used to calculate the contribution of vibrational and rotational energies of hydrogen molecules to the thermal conductivity of heavy species. The transport properties are calculated using the formulas derived by Hirschfelder, Curtiss and Bird. The data of collision integrals or collision cross-sections between species in the mixture are taken from Murphy, Devoto and Mason’s publications. The binary mass diffusion coefficients between the species in the mixture are also calculated from these collision integral data. The mass diffusion of species in the mixture are modelled under the dilute approximation at present since the mole fraction of the principal species, Argon, in the whole computational region is more than 90%. For charged species, Ambipolar diffusion coefficients are used. Mass balance equations are solved to obtain the mass fractions or mole fractions or the number densities of all the species except for electrons. The electron number density is determined by the condition of electrical neutrality. The developed model is applied to the modelling of inductive plasma flow, generated by the Tekna PL-35 torch model, under different pressures and then to the supersonic plasma flow. The model has been validated by comparing the transport properties under the LTE conditions from this model with the corresponding published values.

Induction plasma deposition has been applied in spray coating and near-net shape forming since long. In this paper, we present a few typical results in applying induction plasma spraying technique to fabricate the near-net shape tungsten components. With various shape, very thick, and large surface of W parts were fabricated, the microstructure in the bulk is uniform, and the density is greater than 98% theoretical density.

Various turbulent models (Spalart-Allmaras, Standard k-ε, RNG k-ε, Realizable k-ε and Reynolds Stress Models) along with Standard, and two-zonal wall functions are used to simulate inductively coupled plasma flows. The computational results can be classified into two categories: All the turbulent models that include low Reynolds number effects, such as, Low Reynolds number k-ε model, Spalart-Allmaras one-equation model, Standard k-ε model with two-zonal wall function, RNG model with turbulent viscosity determined by a differential equation, RSM etc., give similar modelling results. These models predict almost the same temperature contours which are similar to the one predicted by laminar model. The viscosity ratios in plasma region predicted by these models are very close to zero except for in the wall-neighbouring cells, which means the plasma flow is almost laminar. The other category contains those models that do not include the low Reynolds number effects, such as Standard, RNG and so-called Realizable k-ε models with standard wall function. They predict the plasma flow to be turbulence-dominated. In comparison with the results of experimentally measured heat fluxes to a substrate, the heat fluxes predicted by these models that include low Reynolds number effect are very close to experimental measurements while these models that do not include low Reynolds number effects deviate greatly from experimental measurements. It is found that the Reynolds stress model(RSM) appears to be the best predictive model.

In this study, two complementary techniques of diagnostic are used to study the properties of a supersonic HF plasma flow: namely, an enthalpy probe measurement and a flow visualization. A PL-35 induction plasma torch operating with three convergent-divergent Laval-type nozzles generates the plasma: a Mach 3.0 velocity water-cooled (wc) nozzle, a Mach 1.5 velocity wc nozzle and a Mach 2.45 velocity radiation-cooled (rc) nozzle. The plasma plate power is fixed at 20 kW and chamber pressure varies between 1 and 10 kPa. The plasma gas is argon and its flow is fixed at 60 slpm. The enthalpy probe profiles of local enthalpy and stagnation pressure are measured, from which, temperature, velocity and Mach number are obtained. The effect of nozzle design on plasma properties is investigated. The RC nozzle creates a plasma jet hotter with a steeper thermal profile and a higher mean velocity than the wc nozzle. The enthalpy probe calculations imply the assumption that the static pressure of the flow is similar to the chamber pressure. Experimental results show that this assumption is still applicable in the jet fringes, but its value changes strongly along the radial and axial axis. Also, photographs of the oblique shock wave in front of a cone in the plasma flow allow the approximation of the flow Mach number produced by a Mach 1.5 velocity wc nozzle. Its approximate value is 2.0, which is higher than predicted.

A thermal cycling test rig and procedure was designed in order to predict the life expectancy of Thermal Barrier Coatings (TBC) under thermal cycling conditions similar to those meet in combustion chambers. Two 2kW-halogen lamps highly focused on the TBC were used to expose the surface of the coating to an intense heat flux. A 25x100 mm TBC is Air Plasma Sprayed (APS) centered onto a substrate 25x370 mm. The thermal cycling can be done either under inert or oxidizing atmosphere in order to separate oxidation-induced acoustic emissions from that resulting from the mismatch of the Coefficient of Thermal Expansion (CTE) of the coating compared to that of the substrate. Two transducers located at each end of the substrate monitor the Acoustic Emission (AE) signals emitted by crack initiation and/or propagation, were recorded and analyzed in order to deduce available information about TBC behavior under thermal load. The use of two transducers with a time of flight approach provides a valuable means of identifying both the crack formation and its location. This thermal cycling test is adequate for the study of various samples, like welded substrates coated with TBC or TBC coated around holes. The presence of cracks is observed using metallography preparation and microscopic observation.

An experimental study of the spheroidization efficiency of induction plasma processes was completed. The main objective being to obtain models which could be subsequently used for the prediction of the spheroidization efficiency for various powders and plasma operating conditions. Silica, alumina, chromium oxide and zirconia powders were treated during the experimentation. For the plasma treatment of the powders the installation used had a maximum available power of 50 kW with an operating frequency of 3 MHz. Operating conditions were varied such to minimize side reactions and the evaporation of powders. The resulting powders did show the presence of cavities and a slight change in the mean diameters. The maximum energy efficiency based semi-empirical model did predict the spheroidization efficiency of the particles beyond a defined critical point known as the maximum energy efficiency point. For the model, the maximum energy efficiency is distinct for the individual powders but remain within a defined range which is reflected in the small variations in the Z constant.

A thermal cycling test was designed in order to predict thermal barrier coatings (TBC) life of combustion chambers. The thermal cycle is produced by two 2kW halogen lamps highly focused on the TBC. A 25X100 mm TBC is plasma sprayed centered onto a substrate 25X300 mm. The thermal cycling can be done either under argon atmosphere or air in order to be able to discriminate the oxidation induced acoustic emissions from the expansion mismatch. Two transducers located at each end of the substrate monitor the acoustic signals emitted by crack initiation and/or propagation. The advantage of using two transducers is that with a time of flight approach cracking phenomena can be located along the TBC. This process allows the study of welded substrates coated with TBC, TBC coated around holes and to check for the presence of cracks by using metallography preparation. The challenge of the test is to use the early cycles emission signatures in order to predict the long term durability of the TBC.

A study on induction plasma shape forming with yttria stabilized zirconia (YSZ) was conducted as part of an effort to develop a new method for producing nuclear fuel. YSZ was selected because its melting point is similar to that of UO2. Nuclear fuel pellets were made using a large (70 mm) induction plasma flame that sprays more than 100 pellets simultaneously and a small (10 mm) supersonic plasma flame that produces one pellet at a time. Process optimization for the large induction plasma flame was done based on chamber pressure, plasma plate power, powder spraying distance, sheath gas composition, probe position, and particle size. The best results were 97.11% theoretical density (TD) for 5-mm thick pellets. For the single pellet approach, densities as high as 99% TD have been obtained in 12-mm thick free-standing pellets.

In this paper, supersonic RF induction plasma deposition of Yttria Stabilized Zirconia (YSZ) has been developed in order to produce dense solid electrolyte membranes for Solid Oxide Fuel Cells (SOFC). Different RF induction plasma torch configurations were tested. The results show that high density layers could be obtained using a supersonic Laval nozzle integrated on a standard torch. 50 to 100 um YSZ coatings with porosity of near 1% could be obtained using this technique at relatively high deposition rates (10g/min.). Attention has been given to the thin coating porosity measurement by using a proper calibration and back scattered electron micrographs of the deposit cross-section coupled with image analysis. Absolute porosity has been measured by using this technique described in another paper of the same conference.

Suspensions of cobalt spinel (Co3O4) powders were rf plasma sprayed to form electrocatalytically active anode layers. Stable cobalt oxide suspensions of low viscosity exceeding 50 wt% solid phase have been processed. A spheroidization study revealed the formation of large spherical powder particles (- 30 + 80 µm). Cobalt oxide coatings were produced by rf suspension plasma spraying. The porosity was controlled by optimizing spray distance and reactor pressure. The main disadvantage of the thermal plasma processing of cobalt spinel is that the decomposition of the spinel phase into CoO could not be prevented, not even with the application of an 80% oxygen plasma. However, with a relatively low power oxygen plasma post-treatment, the deposited CoO layers can be oxidized to Co3O4, greatly improving the electrochemical performance of the anode layers.

This paper describes the procedure that has been developed for absolute porosity measurement using Image Analysis (IA). Because of the crumbly nature of the composite substrate, it was not possible to proceed with standard method. The IA conducted on Optical Microscopy did not show enough contrast between pores and other features to be automated. The IA conducted on Scanning Electron Microscopy (SEM) with back scattered electron imaging gives enough contrast for automatic threshold determination. The SEM magnification is a parameter to be considered because it filters the information. Three frames at 500X magnification are enough for measuring the porosity of homogeneous supersonic induction plasma sprayed 18 mm samples (thickness 50-100 µm). The established calibration almost shows a 1 to 1 ratio for the image analysis as measured porosity versus the Archimedean porosity. Application of this absolute porosity determination by IA can be found in the Functionally Graded Materials (FGM) which composition is not constant over the layer thickness.

In this paper, RF induction plasma deposition of Yttria Stabilized Zirconia (YSZ) is studied with the objective of producing dense solid electrolyte membranes for Solid Oxide Fuel Cells (SOFC). Different RF induction plasma torch configurations were tested. The results show that high density layers could be obtained using a supersonic Laval nozzle attachment on a standard torch. Coating with apparent porosity of less than 1% could be obtained using this technique at relatively high deposition rates. Optical or electron microscopic examination of the deposit coupled with image analysis show, however, that porosity measurements using these techniques can suffer from relatively large discrepancies depending on equipment setting. A discussion is presented of the validity of this porosity measurement technique.

Fine (median size 6 μm and 0.3 μm) cobalt spinel (Co 3 O 4 ) powders were processed suspended in a suitable liquid phase. Suspensions exceeding 50 wt.% solid phase content were successfully injected into an inductively coupled plasma. Spheroidized powders with large particle size (up to 80 μm) were prepared, and cobalt oxide coatings were produced by this novel RF-SPS method. The microstructural features of the coatings can be controlled by parameter optimization similarly to plasma spraying of dry powders. Numerous variations of the physical and chemical conditions of the process were performed in an attempt to overcome the main disadvantage of the process, i.e. the decomposition of the spinel phase to CoO. So far, the spinel phase could be reestablished only by a post-treatment of the deposited coatings with atomic oxygen in the RF plasma.

Suspension Plasma Spraying (SPS) is a thermal spray process based on a suspension of fine (<10 μm) or even ultrafine (<100 nm) powders which is axially fed into the induction plasma through an atomization probe. The atomization of the suspension results in microdroplets (20 μm in size). They are flash dried in the plasma, melted and finally can impact a substrate to build a coating or be cooled down and collected as a spheroidized powder. The large industrial potential of this technology results first from the use of fine powder or even sol-gel which is one of the starting step for many ceramic processes, and second from the various side benefits of the liquid phase in the SPS. Indeed, the liquid phase can be simply a carrier for ultrafine powder, or a protection against oxidation in the case of metals, or a protection for health in the case of whiskers, for instance. It can also take a part in chemical reactions when the liquid phase is a solution of chloride, nitrates... or it can be an organic liquid for the synthesis of carbide, where CO is a strong reducer. Furthermore the liquid phase can also release some energy because of its combustion at the very end of the process. It can also change the local atmosphere surrounding the in flight droplets in the plasma where it is possible to use H 2 O 2 as a carrier in order to increase the oxygen partial pressure around sensitive to oxygen decomposition materials. The applications of SPS are in the powder synthesis (in R&D or production), in the spraying of metals, ceramics or composites directly synthesized, or in production of very reactive with air materials. Applications of SPS will be presented for hydroxyapatite (HA) and NiAlMo. Induction plasma SPS coatings and/or powders properties will be discussed as a function of the SPS process variables.

A study is carried out of the spheroidization of ceramic and metallic powders using induction plasma technology. The process is based on the central injection of the powder in the plasma discharge followed by the in-flight cooling and solidification of the molten droplets prior to their collection at the bottom of a stainless-steel water cooled chamber. The degree of spheroidization is evaluated using image analysis. The results are correlated as a function of the powder feed rate, the plasma operating conditions and the thermophysical properties of the powders treated. The model's fit to the obtained experimental data is very good. The results show that the technology can be successfully used for the spheroidization and densification of a wide range of materials.